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New Study Presents Nature-Inspired Novel Catalyst for Efficient Hydrocarbon Decomposition

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The findings of this research have been published in the online version of Journal of the American Chemical Society (JACS) on June 3, 2024.
Abstract
A series of mononuclear manganese(III)–hydroxo and −aqua complexes, [MnIII(TBDAP)(OH)2]+ (1), [MnIII(TBDAP)(OH)(OH2)]2+ (2) and [MnIII(TBDAP)(OH2)2]3+ (3), were prepared from a manganese(II) precursor and confirmed using various methods including X-ray crystallography. Thermodynamic analysis showed that protonation from hydroxo to aqua species resulted in increased redox potentials (E1/2) in the order of 1 (−0.15 V) < 2 (0.56 V) < 3 (1.11 V), while pKa values exhibited a reverse trend in the order of 3 (3.87) < 2 (11.84). Employing the Bordwell Equation, the O–H bond dissociation free energies (BDFE) of [MnII(TBDAP)(OH)(OH2)]+ and [MnII(TBDAP)(OH2)2]2+, related to the driving force of 1 and 2 in hydrogen atom transfer (HAT), were determined as 75.3 and 77.3 kcal mol–1, respectively. It was found that the thermodynamic driving force of 2 in HAT becomes greater than that of 1 as the redox potential of 2 increases through protonation from 1 to 2. Kinetic studies on electrophilic reactions using a variety of substrates revealed that 1 is only weakly reactive with O–H bonds, whereas 2 can activate aliphatic C–H bonds in addition to O–H bonds. The reaction rates increased by 1.4 × 104-fold for the O–H bonds by 2 over 1, which was explained by the difference in BDFE and the tunneling effect. Furthermore, 3, possessing the highest redox potential value, was found to undergo an aromatic C–H bond activation reaction under mild conditions. These results provide valuable insights into enhancing electrophilic reactivity by modulating the redox potential of manganese(III)–hydroxo and −aqua complexes through protonation.

A research team, affiliated with UNIST has developed a novel catalyst that mimics the ability of a natural enzyme to break down harmful hydrocarbons, paving the way for a more environmentally friendly and energy-efficient approach to reducing pollution.

Led by Professor Jaeheung Cho in the Department of Chemistry at UNIST, the research team has succeeded in creating a new catalyst that mimics the ability of metalloenzyme, which are ubiquitous in nature, that is to oxidize hydrocarbons. By adding hydrogen ions to the hydroxo ligand, metal-bound water molecules were synthesized. The new catalyst, which uses metal-bound water molecules, can oxidize carbon-hydrogen (C-H) bonds at lower temperatures than existing methods, making it a game-changer for the development of sustainable technologies.

The addition of hydrogen ions to the manganese catalyst has significantly improved its ability to activate oxygen-hydrogen bonds, resulting in accelerated reaction rates. This enhanced activity is attributed to the increased reduction potential of manganese, achieved by modifying the hydroxyl ligand to include water.

Figure 1. Schematic image, showing the difference in reactivity between a manganese(III)–hydroxo and a manganese(III)–aqua complex.

The new catalyst demonstrated its efficacy in oxidizing anthracene, a substance with strong carbon-hydrogen bonds, at low temperatures, effectively removing toxicity. Additionally, it was able to decompose aromatic hydrocarbons that are insoluble in water and chemically stable.

According to Professor Cho, “This is the first instance where a manganese(III) complex with two water molecules has reacted with aromatic hydrocarbons at low temperatures.” He emphasized that controlling the reduction potential of manganese will contribute to the development of industrially important metal catalysts capable of breaking down strong carbon-hydrogen bonds.

The findings of this research have been published in the online version of Journal of the American Chemical Society (JACS) on June 3, 2024. The study received support from the National Research Foundation of Korea (NRF) and through the Source Technology Development (R&D) for DACU and the National Drug Development Project.

Journal Reference
Yuri Lee, Dohyun Moon, and Jaeheung Cho, “Controlling Redox Potential of a Manganese(III)–Bis(hydroxo) Complex through Protonation and the Hydrogen-Atom Transfer Reactivity,” J. Am. Chem. Soc., (2024).

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